SST: Secure storage implementation code

First revision. Contains:
 -- Flash device independent secure storage solution
 -- Policy based access control
 -- HUK based encryption
 -- AEAD encryption policy (AES-GCM) that ensures
    -- Content integrity
    -- Content privacy
    -- Localization due to HUK based encryption
 -- Power failure safe update(write) operation

Change-Id: I18f6179732cb1fbdcf902306c39df3ef0c845488
Signed-off-by: Abhishek Pandit <abhishek.pandit@arm.com>
Co-Authored-By: Vlad Ellis <vlad.ellis@arm.com>